Arendse, L.M.

Gianakouras, D.

Giddey, C.J.

Hashoongo, V.

Herbert, V.

Jones, S.

Kemp, A.D.

Maritz, B.

Rogers, C.

Ruby, C.

Verheye, H.M.

Open Marine Archive

       
      

 
 
 
 
    
   
 
      
 
 
 
 
 
 
 
      
    
 
 
        
Variability of zooplankton in the region
of the Angola-Benguela Front during
winter 1999
H.M. Verheye , C. Rogers , B. Maritz , V. Hashoongo , L.M. Arendse ,
D. Gianakouras , C.J. Giddey , V. Herbert , S. Jones , A.D. Kemp and
C. Ruby
There is growing recognition that frontal
systems are regions of high productivity
and influence the distribution of biota
across most of the trophic spectrum.1,2 The
Angola-Benguela Front (ABF) is a com-
paratively little studied frontal system at
about 14–17°S off southern Angola. This is
a seasonally highly variable hydrographic
feature separating the Angola and
Benguela currents. These current systems
are characterized by different physical,
chemical and biological properties (see
various contributions elsewhere in this
issue). Information on how physical and
biological processes in the region of the
ABF interact and influence biological pro-
duction and the spatial distribution of
biota is scarce. Little is known, for in-
stance, about the spatial variability and
taxonomic composition of zooplankton in
this frontal boundary region.
As part of the 1999 BENEFIT Shipboard
Research Training Programme in the
SADC Region,3 zooplankton samples
were collected from the Namibian RV
Welwitchia using a 200-µm meshed
Bongo-net system, towed obliquely in the
upper 200 m between Walvis Bay (23°S)
and Luanda (9°S) during 24 July–1 August
1999 (see Fig. 5 in ref. 3). The samples were
analysed during a BENEFIT Zooplankton
Identification Training Workshop† and re-
vealed distinct longshore and cross-shelf
patterns in numerical abundance, distri-
bution and taxonomic composition of zoo-
plankton. These patterns were associated
with different water masses, identified
from sea-surface temperature (SST) data,
encountered along the cruise track
(Fig. 1a).
Stations within the ABF region (SST:
18–21°C) were characterized by elevated
abundances of total zooplankton (Fig. 1a),
on average 93.0 ± 17.3 × 103 ind. m–2
(mean ± 1 s.e.), compared to the warm
Angola Current waters (22–24°C) to the
north with 33.9 ± 8.7 × 103 ind. m–2 and
the cool Benguela Current waters
(14–17°C) to the south with 28.8 ± 6.7 ×
103 ind. m–2.
Throughout the survey area, calanoid
copepods were the most abundant, com-
prising roughly between 50 and 60% of
the total zooplankton (Fig. 1b). Cyclopoid
copepods were relatively more abundant
in the Angola Current region (14%) than
elsewhere (5–6%). Gelatinous zooplank-
ters were more abundant in the Benguela
Current region off Namibia (19%) than
in the Angola Current region (5%), and
represented only 2% of the total zoo-
plankton within the ABF. Euphausiids
made up 26% of the zooplankton in the
latter region, while only 14% to the north
of it and 12% to the south. Chaetognaths
were fairly uncommon, making up
merely 3–8% of total zooplankton abun-
dance. Other taxa, which included clado-
cerans, polychaetes and some other, less
common groups represented between 5
and 11% of the total abundance in the
respective regions.
A 90-n mi transect along the 20°S
aMarine and Coastal Management, Private Bag X2,
Rogge Bay,8012 South Africa.
bDepartment of Nature Conservation and Oceanography,
Cape Technikon, P.O. Box 652, Cape Town, 8000 South
Africa.
cMinistry of Fisheries and Marine Resources, National
Marine Information and Research Centre, P.O. Box 912,
Swakopmund, Namibia.
dMinistério das Pescas e Ambiente, Instituto de
Investigação Marinha, Ilha de Luanda, C.P. 2601,
Luanda, Angola.
*Author for correspondence.
E-mail: hverheye@mcm.wcape.gov.za
latitude off northern Namibia provided
an opportunity to investigate cross-shelf
distribution patterns of zooplankton
(Fig. 2) . There was a marked in-
shore–offshore trend in total zooplankton
abundance, with lowest numbers associ-
ated with the cooler (<15°C) shelf waters,
on average 12.7 ± 2.2 × 103 ind. m–2, in-
creasing to 49.0 ± 4.9 × 103 ind. m–2 in
warmer (15–17°C) water over the slope,
and reaching a distinct peak of 747.1 × 103
ind. m–2 in deep oceanic water at the out-
ermost station. Of the crustacean fraction,
copepods dominated the shelf commu-
nity (66%), whereas euphausiids were the
most abundant in the slope region (59%).
By contrast, gelatinous zooplankters were
most prominent at the outermost station
(77%), where salps accounted for the
highest zooplankton density observed
during the cruise. Although much less
abundant than the aforementioned
groups, chaetognaths (11%) and ‘other
taxa’ (14%) were comparatively far more
important on the shelf than further off-
shore (<2%), a trend similar to that ob-
served for the copepods.
This study, although based on sample
analyses at a coarse taxonomic resolution,
illustrates the distinct association of zoo-
plankton communit ies with the
hydrography of the Angola and Benguela
current systems. In addition to cross-shelf
variability, there were marked changes in
zooplankton species composition and
abundance across the ABF. The impor-
tance of the ABF frontal boundary in sup-
porting increased zooplankton levels is
highlighted, with especially crustaceans
being able to maintain high population
densities, likely by combining behav-
ioural traits with specific features of the cir-
culation system. It is suggested that the
ABF is a site of either physical accumula-
tion or convergence of planktonic organ-
isms, or enhanced productivity, or both. A
more rigorous assessment of spatial gra-
dients in the abundance of zooplankton
at both the species and developmental
stage levels may provide more clarity on
these hypotheses.
†Sponsored by the BENEFIT Programme, the work-
shop was convened by Hans M. Verheye of Marine
and Coastal Management Coordination (M&CMC) in
Cape Town, South Africa and held at the National
Marine Information and Research Centre (NatMIRC)
in Swakopmund, Namibia, during 18–25 September
1999. It was a continuation of the training activities
conducted during the BENEFIT cruises. Bringing
together marine scientists and oceanographic techni-
cians from Angola, Namibia and South Africa, the
workshop aimed at contributing further to capacity
building in marine science, particularly zooplankton
research, in the Benguela Current region. We wish to
thank BENEFIT, M&CMC and NatMIRC for financial
and logistical support, as well as the Master and crew
of the RV Welwitchia for their assistance at sea.
1. Armstrong D.A., Mitchell-Innes B.A., Verheye-
Dua F., Waldron H. and Hutchings L. (1987). Phys-
ical and biological features across an upwelling
front in the southern Benguela. In The Benguela
and Comparable Ecosystems, eds A.I.L. Payne, J.A.
Gulland and K.H. Brink. S. Afr. J. mar. Sci. 5,
171–190.
2. Hutchings L., Armstrong D.A. and Mitchell-Innes
B.A. (1986). The frontal zone in the southern
Benguela Current. In Marine Interfaces Ecohydro-
dynamics, ed. J.C.J. Nihoul, pp. 67–94. Elsevier
Oceanography Series 42, Amsterdam.
3. Hocutt C.H. and Verheye H.M. (2001). BENEFIT
marine science in the Benguela Current region
during 1999: Introduction. S. Afr. J. Sci. 97, 195–198.
258 South African Journal of Science 97, May/June 2001 BENEFIT Marine Science
Fig. 1. a, Longshore distribution and depth-integrated total abundance (No. m–2) of zooplankton in the upper
200 m in the region of the northern Benguela and Angola-Benguela Front, with sea-surface temperature super-
imposed showing three distinct water masses. The first four digits of the station numbers refer to latitudinal
station position (XX°YY’S), the last two indicate distance offshore in nautical miles; b, mean taxonomic composi-
tion (%) of zooplankton in the samples pooled for each of the three water masses.
Fig. 2. Bottom, cross-shelf distribution and depth-integrated total abundance (No. m–2) of zooplankton in the
upper 200 m along a transect at 20°S off Namibia, with sea-surface temperature superimposed; Top, mean
taxonomic composition (%) of zooplankton in samples pooled for shelf, slope and deep oceanic stations.


Variability of zooplankton in the region of the Angola-Benguela Front during winter 1999

Variability of zooplankton in the region of the Angola-Benguela Front during winter 1999

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